1. Field of the Invention
The present invention relates to a cleaning member, a charging device, a transfer device and an image forming apparatus used for a printer, a copying machine, or the like.
2. Description of the Related Art
In general, an image forming process in an electrophotographic image forming apparatus using a toner as a developing agent comprises mainly the following steps:                (1) preliminary exposure of an image carrier (photosensitive member) (this step can be omitted when an alternating current voltage is applied to a direct current voltage so as to be superimposed thereon, as has been carried out recently),        (2) charging the image carrier surface,        (3) forming a latent image by exposure,        (4) forming a visual image using toner,        (5) transferring the toner image onto a transfer material (paper, or the like),        (6) cleaning the image carrier surface after the transfer, and        (7) fixing the toner image on the transfer material by heating, or the like.        
Recently, as an important issue for electrophotographic image forming apparatuses, attention has been paid to making the life of the apparatuses longer.
This is aimed at reducing the film reduction of the image carrier disposed in the center of the electrophotographic image forming apparatus so as to maintain good image formation for a longer time. This in turn is greatly effective in reducing operating cost.
It is known that the charging mechanism is greatly influenced by film reduction of the image carrier. Types of the charging mechanism vary widely. For example, contact charging, which has been in use for about 10 years, is for applying a charging bias by rotatingly a charge roller which comprises an elastic member with resistance adjusted so as to be semi conductive (about 105 Ωcm),in contact with the image carrier. The charge bias at a time when an alternating current voltage having an inter-peak voltage of at least two times as high as a discharge starting voltage of a direct current is superimposed and applied to a direct current voltage. Due to the application of the alternating current voltage, the potential of the image carrier surface is converged to the applied direct current voltage value, and as a result, uniform charging of the image carrier surface can be achieved.
Moreover, since the above-described contact charging utilizes pulse discharge in a minute space between the image carrier and the charge roller, a generated amount of ozone, which is harmful to human bodies, can be kept to an extremely small amount compared with a corotron charge, which was used previously, and drastic reduction in a cost of the charge member can be achieved. Thus, contact charging is now in the mainstream of charging methods.
However, since contact charging changes the image carrier surface by utilizing pulse discharge as described above, the image carrier surface is always in an etched state. This results in a harmful effect in that the film reduction of the image carrier is further accelerated.
Therefore, in order to reduce the film reduction of the image carrier by restraining the pulse discharge, the following novel charging mechanisms have been proposed.
(1) Direct Current (DC) Charging (a Method of Applying only a Direct Current Voltage to the Contact Charge Roller)
This method is for charging the image carrier surface only using direct current voltage. Since alternating current voltage is not applied, the amount of current flowing to the image carrier is extremely small. That is, the pulse discharge to the image carrier is reduced as well. As a result, etching with regard to the image carrier (photosensitive member) is reduced, and the film reduction of the image carrier is kept to a smaller amount.
(2) Non-contact Charging (a Method of Applying an Alternating Current Voltage Superimposed on a Direct Current Voltage while Providing a Certain Gap between the Image Carrier and the Charge Roller)
Compared with contact charging (when applying an alternating current voltage superimposed on a direct current voltage), this method has an extremely small inflow of current to the image carrier. In contrast to a current generated by a nip of the charge roller and the image carrier and a discharge current accompanying a pulse discharge generated in a gap in a range satisfying the Paschen's discharge start voltage existing on the right and left sides of the nip by the contact charge, by providing a certain gap between the image carrier and the charge roller the nip current generation can be prevented and the pulse discharge is generated only at the shortest part of the gap between the image carrier and the discharge roller. Thus, etching with respect to the image carrier can be reduced so that the film reduction of the image carrier can be kept to a small amount.
However, the above-described charging mechanisms respectively have the following problems.
(1) Direct Current (DC) Charging
The amount of discharge current in DC charging is determined by a resistance value of the contact charge roller, so that the pulse discharge state depends largely on the properties of a surface of the contact charge roller. Therefore, in order to uniformly charge only by applying a direct current voltage on the image carrier surface, compared with the case of applying an alternating current voltage superimposed on direct current voltage, a further even electric resistance and surface smoothness are required. Therefore, in order to improve the electric resistance evenness and the surface smoothness, cost increase cannot be avoided.
Moreover, according to the DC charging, contamination (transfer toner, paper dusts, or the like) can easily adhere to the charge roller surface. Since the electric resistance and the surface condition are varied by the adhered contamination, the uniform charging can easily be inhibited. This is considered that the contamination adhered on the charge roller can hardly be eliminated due to absence of the oscillating electrolysis because the alternating current voltage is not applied.
Therefore, in order to maintain a exellent image over a long time while reducing the film reduction of the image carrier according to the DC charging, a cleaning mechanism for the charge roller or a mechanism for completely preventing adhesion of contamination on the charge roller is indispensable.
With respect to the above-described problems, a method of forming the surface layer of the charge roller with a high mold releasing property material, a method of applying an inverse bias which is polarity opposite to the charge polarity every certain period of time for inverting the polarity of the residual toner so as to be moved onto the image carrier, or the like have been attempted. However, the mold releasing property of the surface is lowered as time passes by as well as the effect of the inverse bias is lowered, so that a high image quality cannot be provided over a long time in the present situation.
(2) Non-contact Charging
According to the non-contact charging, since a certain gap is provided between the image carrier and the charge roller, the applied bias necessary for charging should be increased. This is apparent from the approximate expression of the Paschen's discharge start voltage: VTH=312+6.2×Z (Z is gap amount (μm)).
The increase of the applied bias brings about increase of the ozone generating amount, so that the amount of the discharge product (NOx, or the like) adhered on the image carrier surface is increased.
Moreover, in the case of applying an alternating current voltage superimposed on a direct current voltage in the non-contact charging, since the alternating current applied bias is naturally increased as well, the potential difference between the charging member surface and the image carrier surface is enlarged. Thereby, the charging member can easily attract the transfer residual toner, the paper dusts, or the like on the image carrier. As a result, the contamination accumulation amount on the charging member tends to be increased.